1. Field of the Invention
The present invention relates to a combustion burner used in industrial heating furnaces and a combustion method thereof in furnace.
2. Description of the Related Arts
It is well known that the main factors affecting the creation of NOx in exhaust gas is a combustion temperature and an oxygen concentration and the higher is the combustion temperature or the higher is the oxygen concentration, the higher is the NOx concentration in the exhaust gas. Although many methods of suppressing the creation of the NOx gas have been proposed for many years ago, they are roughly classified as a method of changing combustion conditions and a method of changing a combustion method itself.
The former method is a suppression method by (1) combustion with a small amount of oxygen (combustion by a low air ratio), (2) reduction of heat load in a combustion chamber (combustion at low temperature), (3) reduction of combustion air temperature (preheated air temperature), and the like. The latter method is a suppression method by (4) a two-stage combustion method, (5) a off-stoichiometric combustion method, (6) an exhaust gas recycling combustion method, (7) a steam blowing combustion method, and the like. Some of the methods cannot be applied as a method of suppressing NOx gas from the view point of heat efficiency because they lower the heat efficiency.
A practically used combustion burner for lowering NOx concentration (hereinafter, referred to as a low NOx burner) basically incorporates the functions of the items (4)-(7) of the latter method compactly to realize the reduction of NOx as an overall effect while executing combustion at a low air ratio. For example, Japanese Examined Patent Publication No. 56-8921 and Japanese Examined Patent Publication No. 56-8922 disclose a combustion method embodying these functions (FIG. 6). Further, recently, Japanese Unexamined Patent Publication No. 1-167591, Japanese Unexamined Patent Publication No. 1-300103 (FIG. 7), Japanese Unexamined Patent Publication No. 7-4612, Japanese Unexamined Patent Publication No. 7-4613 (FIG. 8) and the like disclose a combustion method of further lowering the concentration of NOx in exhaust gas by directly blowing fuel into a heating furnace.
Low NOx combustion will be described with reference to FIG. 6 to FIG. 8.
First, the combustion burner of FIG. 6 will be described. A primary fuel nozzle 20 is disposed to an air introduction pipe 22 and secondary fuel nozzles 21 are disposed on both the sides of a precombustion chamber 23. Combustion air is revolved by an air revolving blades 4, mixed with fuel injected from the primary combustion nozzle 20 and precombusted in the precombustion chamber 23. Further, fuel from the secondary fuel nozzles 21 is combusted in a main combustion chamber 25. NOx created by the combustion of the fuel supplied from the secondary fuel nozzles 21 into the main combustion chamber 25 can be greatly reduced by the two-stage combustion method.
The combustion burner of FIG. 7 is arranged such that a primary fuel nozzle 20 is disposed to a combustion air supply port 22 for use in low temperature combustion and secondary fuel nozzles are disposed around the combustion air supply port 22. Exhaust gas in a furnace is rolled in by combustion air and fuel and recombusted to lower NOx.
The alternate burner 30 of FIG. 8 is arranged such that a primary combustion nozzle 28 is provided with each pair of regenerative burners 27 and a secondary combustion nozzle 31 carries out secondary combustion by the recirculation thereof while the primary combustion nozzles 28 carry out combustion at low temperature.
However, since the combustion method of the combustion burner of FIG. 8 is a combustion method of directly blowing fuel into a heating furnace, although it is effective for suppressing the creation of NOx, the combustion burner includes the primary fuel nozzle for low temperature combustion in the air supply port thereof to start up the furnace. Thus, there is a tendency that a fuel supply system and other piping system are made complex and further a cooling air pipe is needed to cool a fuel supply port in a high temperature furnace. Consequently, not only the piping system for cooling air and the like is made complex and an initial equipment cost is made expensive but also the piping system for fuel and combustion air, the control system therefor and the improvement of a heating furnace, for example, are required to improve the performance of the heating furnace. However, since the piping system and the like are complex, there is a problem that the improvement is actually limited to a slight degree, thus there is room for improving the combustion burner and the combustion method thereof in the furnace to lower NOx.
In the above low NOx combustion methods, the low NOx combustion burners of FIG. 6 and FIG. 7 are advantageous because they are relatively simple in structure and the performance of the heating furnace can be easily improved only by replacing the combustion burner. However, the combustion methods achieved by the burners are inferior to the combustion method of directly blowing fuel into the furnace shown in FIG. 8 as to the effect for suppressing the creation of NOx. In particular, when an air preheating temperature is increased for the purpose of saving energy, there is a danger that the low temperature combustion of FIG. 6 and FIG. 7 is made to a high temperature combustion state and exceeds a NOx regulating value. Further, since the width of a heating furnace is an important factor for lowering NOx in the combustion burner of FIG. 7, there is a defect that the width of the heating furnace must be improved in accordance with the combustion burner when the burner is employed.